It is standard practice throughout the molded case circuit breaker industry to individually calibrate each thermal magnetic trip unit within such breakers during the manufacturing process. Those breakers failing to meet specified response characteristics are rerouted for additional calibration. The recalibration, often manually performed, interferes with the overall efficiency of circuit breaker manufacture.
Calibration is also required with static trip circuit breakers containing electronic trip circuits. This calibration is undertaken upon computerized test equipment wherein circuit trimming is automatically achieved in response to computerized determinations. The primary sources of errors within electronic trip units are the current transformers and the analog-to-digital conversion circuits. A further source of error lies in the gain set resistors used with operational amplifiers contained within the signal processor network within the trip circuit. Since the transfer characteristics of each trip unit module must meet the specification requirements set by industry standards, some means of adjustment of the gain set resistors must be externally employed without deterring from the automatic assembly of the module during its final stages of assembly.
One example of an electronic trip unit for static trip circuit breakers is found within U.S. patent application Ser. No. 631,708, filed July 17, 1984, entitled "Digital I.sup.2 T Pickup, Time Band and Timing Control Circuit For Static Trip Circuit Breakers" in the name of John Dougherty. Most of the circuit components are located on an integrated circuit which could also include a digital processor. This application is incorporated herein for purposes of reference.
U.S. Pat. No. 4,550,360, entitled "Circuit Breaker Static Trip Unit Having Automatic Circuit Trimming" also in the name of John Dougherty, describes means for trimming those circuits which do include a digital processor.
One purpose of the instant invention is to provide means for automatic circuit trimming of the gain set resistors to calibrate an electronic trip unit of the type employing a digital logic circuit without a digital processor.